1. Field of the Invention
This invention relates to a reception electric field level detection circuit for a mobile communication system which employs a time division multiplexing method in which a plurality of mobile radio units make use of a radio channel in a multiplexed condition with respect to time.
2. Description of the Prior Art
A mobile communication system is known wherein a plurality of mobile radio units send out carriers to a plurality of stationary radio units using a plurality of radio channels which use frequency bands adjacent to each other. In a mobile communication system of the type mentioned, a reception electric field level value of a carrier received by a stationary radio unit is detected to discriminate the position of the transmission side (mobile radio unit). The detection of a reception electric field level value is performed by a reception electric field level detection circuit provided in each stationary radio unit.
FIG. 1 is a block diagram showing a conventional reception electric field level detection circuit for a mobile communication system which employs the time division multiplexing method.
The conventional reception electric field level detection circuit shown includes frequency conversion circuit 11, IF filter 12 and demodulation circuit 13 for outputting a demodulation signal, which are successively connected to antenna 17, and reception electric field detection circuit 14 for receiving the output of IF filter 12 and detecting the reception electric field level value of the received radio wave. It is to be noted that IF filter 12 has a flat characteristic within its reception band.
Frequency conversion circuit 11 receives a RF signal from antenna 17 as an input thereto and outputs an IF signal to IF filter 12. IF filter 12 has a frequency selection property of passing the band of a desired wave freely therethrough but attenuating any other band than that of the desired wave by a great amount in accordance with an attenuation factor curve, and passes a signal of a predetermined region therethrough. Demodulation circuit 13 and reception electric field detection circuit 14 receive the output of IF filter 12 as inputs thereto and output a demodulation signal and a reception electric field level value, respectively, in response to the thus inputted signal.
Next, detection of a reception electric field level value from a RF signal including an adjacent channel wave by the reception electric field level detection circuit described above is described.
FIG. 2(a) is a graph showing the RF signal including an adjacent channel wave and the band of IF filter 12 and indicates that the electric field level of the adjacent channel wave is higher than the electric field level of the desired wave. Meanwhile, FIG. 2(b) is a graph showing the output of IF filter 12 in FIG. 2(a) and indicates that, similarly to FIG. 2(a), the electric field level of the adjacent channel wave is higher than the electric field level of the desired level.
In the reception electric field level detection circuit shown in FIG. 1, when the RF signal shown in FIG. 2(a) is inputted from antenna 17 to frequency conversion circuit 11, the IF signal shown in FIG. 2(b) is outputted from IF filter 12. The thus outputted IF signal is inputted to demodulation circuit 13 and reception electric field detection circuit 14.
In the case described above, the IF signal inputted to reception electric field detection circuit 14 exhibits an electric field level value higher at the adjacent channel wave than at the desired wave as seen from FIG. 2(b). Therefore, reception electric field detection circuit 14 detects the electric field level value of the adjacent channel wave as a reception electric field level value of the IF signal. Accordingly, the conventional reception electric field level detection circuit described above cannot detect the position of the transmission side (mobile radio unit) accurately.
Thus, another reception electric field level detection circuit has been proposed wherein an additional IF filter is provided in the input line to the reception electric field detection circuit 14 in order to completely remove an adjacent channel wave from an IF signal inputted to the reception electric field detection circuit 14.
FIG. 3 is a block diagram showing the conventional reception electric field level detection circuit which completely removes an adjacent channel wave.
The reception electric field level detection circuit shown in FIG. 3 is similar to the reception electric field level detection circuit of FIG. 1 except that IF filter 16 is provided in the line branched from the output of IF filter 12 and connected to the input of reception electric field detection circuit 14. It is to be noted that, since IF filter 16 has a flat characteristic within its reception band and since the output of IF filter 16 is not used as an input signal to demodulation circuit 13, the bandwidth of IF filter 16 is restricted narrow so as to completely remove an adjacent channel wave.
Operation of the reception electric field detection circuit when it receives a RF signal including an adjacent channel wave is described below.
FIG. 4(a) is a graph showing the IF signal outputted from IF filter 12 and the band of IF filter 16 and indicates that the electric field level of the adjacent channel wave is higher than the electric field level of the desired wave. Meanwhile, FIG. 4(b) is a graph showing the output of IF filter 16 in FIG. 4(a) and indicates that the adjacent channel wave of FIG. 4(a) is removed completely and also the band of the desired wave is partially removed.
In the reception electric field level detection circuit shown in FIG. 3, when a RF signal shown in FIG. 2(a) is inputted from antenna 17 to frequency conversion circuit 11, an IF signal shown in FIG. 4(a) is outputted from IF filter 12 similarly as in the reception electric field level detection circuit shown in FIG. 1. The thus outputted IF signal is branched and is inputted, on one hand, to demodulation circuit 13 and, on the other hand, to IF filter 16.
Demodulation circuit 13 demodulates the thus inputted IF signal and outputs a demodulation signal. Meanwhile, IF filter 16 completely removes the adjacent channel wave from the inputted IF signal and outputs the IF signal shown in FIG. 4(b) to reception electric field detection circuit 14. As a result, only the reception electric field level value of the desired wave is detected by reception electric field detection circuit 14.
Since the reception electric field level value of the desired wave is detected by reception electric field detection circuit 14 in the manner described above, the position of the transmission side (mobile radio unit) can be determined accurately.
However, the conventional reception electric field level detection circuits described above have the following problems.
Of the conventional reception electric field level detection circuits, the reception electric field level detection circuit shown in FIG. 1, which includes an IF filter disposed at a stage, has a problem in that it cannot detect the electric field level of an object channel accurately and cannot discriminate the position of the transmission side (mobile radio unit) accurately.
The reception electric field level detection circuit shown in FIG. 3, which includes IF filters disposed at two stages, includes the additional IF filter inserted in the input line to the reception electric field detection circuit, and accordingly, is required to provide a matching circuit, a buffer amplifier and so forth on the front side and the rear of the additional IF filter. Consequently, the reception electric field level detection circuit has a problem in that the circuit scale is increased and the circuit becomes expensive.